A method for predicting competitive fracturing of multiple fractures during temporary plugging and fracturing of horizontal wells in shale reservoirs

Abstract

The invention discloses a method for predicting competitive fracturing of multiple fractures in temporary plugging and fracturing of horizontal wells in shale reservoirs, comprising: establishing a two-dimensional hydraulic fracturing fracture expansion model of a shale reservoir composed of natural fractures, hydraulic fractures and rock matrix; analyzing the competitive fracturing mechanism of multiple fractures in hydraulic fracturing of the shale reservoir and the influencing factors of fracture network expansion based on the hydraulic fracturing expansion model, and analyzing and obtaining the relationship between the sequence of multiple fractures initiation, the complexity of fracture network expansion and the difference in reservoir ground stress, the fracture cluster spacing and the number of main fracture clusters; using the model, the optimal design values ​​of the number of perforation clusters and the perforation cluster spacing under different stress conditions can be obtained to guide the design of a temporary plugging and fracturing scheme on site; according to the expected fracture morphology, the fracture expansion length, depth and effective transformation volume of the fracture network are obtained, and then the fracturing construction parameters are determined.

Description

Technical Field

[0001] The invention relates to the technical field of hydraulic fracturing for shale gas development, and in particular to a method for predicting competitive fracturing of multiple fractures in temporary plugging and fracturing of horizontal wells in shale reservoirs. Background Art

[0002] As a very important unconventional oil and gas resource, the economic and efficient exploitation of shale gas resources is of great significance to my country's economic and social development. my country has abundant shale gas resources, thick reservoirs and excellent quality, but its deep burial depth, low porosity and low permeability greatly increase the difficulty of its development. At present, the development of shale gas mainly adopts the horizontal well staged fracturing development process, and multi-cluster fracturing within the segment has become the main body of the development process. Its basic principle is to increase the number of perforation clusters within the segment, adopt dense cutting within the segment, increase the number of main fractures, and improve the availability of the reservoir. If dense cutting segmentation is widely adopted, multi-cluster perforation within the segment, and the lower main fracture spacing will increase the interference between fractures, causing some main fractures to be suppressed and unable to fracture. Due to stress interference, the expansion path of the remaining fractures is difficult to predict, and the fracture morphology is complex. In order to make each main fracture cluster expand evenly, temporary plugging fracturing has become a key technology for shale gas development. The on-site fracturing practice in the Changning shale gas block in southern Sichuan shows that the fracture network morphology is complex during temporary plugging and fracturing, the competitive fracturing sequence and expansion mechanism of each main fracture are unclear, and improper setting of fracturing parameters will cause some main fracture clusters to fail to fracture or have a tortuous expansion path, which cannot extend to the maximum extent inside the reservoir. Therefore, before temporary plugging and fracturing of shale reservoirs, it is necessary to predict the competitive fracturing and fracture expansion morphology of the main fractures in order to formulate reasonable fracturing parameters.

[0003] The expansion of fractures in heterogeneous shale reservoirs is affected by both reservoir parameters and fracturing operation parameters, and the horizontal stress difference, the number of perforation clusters and the spacing between perforation clusters have the most significant effects. In recent years, domestic and foreign scholars have conducted relevant research on this and have gained certain understanding. Weng et al. found that when multiple clusters of fractures expand simultaneously, the middle fracture is suppressed while the fractures on both sides expand to a greater depth in the reservoir; Cheng et al. believed that the uneven expansion of multiple clusters of fractures was mainly caused by stress interference; Wu and Olson et al. found that the uneven expansion between main fractures was mainly caused by the uneven dynamic distribution of the fracturing fluid inflow into the main fractures; Zhou et al. studied the fracturing fluid inflow into each main fracture under stress interference and found that the interference stress of the outer fracture expansion would cause the inflow pressure of the middle fracture to increase sharply; Gramer et al. believed that the fracturing fluid friction resistance could be reduced by increasing the perforation diameter of the middle perforation cluster, and the inflow volume could be increased, thereby increasing the extension depth of the middle fracture; Wu et al. found that the expansion of the outer fracture would be suppressed due to the horizontal stress difference; Lecampion et al. found that the development of natural fractures in the near-wellbore area and stress interference caused the main fracture to be too tortuous in the near-wellbore area, and the tortuosity was too high, which increased the inflow pressure of the main fracture; Zhou Tong et al. studied the flow distribution mechanism determined by perforation friction resistance and multi-fracture interference stress. In general, previous studies on the competitive fracturing of multiple fractures in shale reservoirs by hydraulic fracturing have mostly focused on the qualitative description of the influencing factors of different factors on fracture initiation, and have not formed a means of predicting the competitive fracturing of fractures and a quantitative standard for evaluating the fracture network morphology to guide the actual temporary plugging and fracturing of horizontal wells on site. Summary of the invention

[0004] The purpose of the present invention is to address the above shortcomings and propose a numerical simulation study on the expansion of complex fracture networks in temporary plugging fracturing of horizontal wells in shale reservoirs, accurately predict the competitive fracturing sequence and fracture morphology of multiple fractures in temporary plugging fracturing, formulate reasonable fracturing construction parameters, and form a new method for predicting the competitive fracturing of multiple fractures and the expansion of complex fracture networks in temporary plugging fracturing of horizontal wells in shale reservoirs.

[0005] In order to achieve the above object, the present invention specifically adopts the following technical solutions:

[0006] A method for predicting competitive fracturing of multiple fractures in temporary plugging and fracturing of horizontal wells in shale reservoirs, specifically comprising:

[0007] Establish a two-dimensional horizontal well fracturing complex fracture network expansion model consisting of natural fractures, hydraulic fractures and rock matrix;

[0008] Based on the horizontal well hydraulic fracturing fracture propagation model, the factors affecting the competitive fracturing and fracture network expansion of shale reservoir fracturing are analyzed, including the influence of interference stress between fracture clusters, the main fracture spacing, the number of main fracture clusters, and the horizontal stress difference;

[0009] The relationship between the initiation sequence of multiple fractures, the complexity of fracture network expansion, the horizontal stress difference of the reservoir, the spacing between main fractures and the number of main fracture clusters was analyzed.

[0010] The relationship between the sequence of multiple fracture initiation, the complexity of fracture network expansion, the horizontal stress difference of the reservoir, the spacing between main fractures and the number of main fracture clusters is used as a prediction method for competitive fracturing and complex fracture network expansion of horizontal wells in shale reservoirs.

[0011] Furthermore, the relationship between the multiple fracture initiation sequence, the complexity of fracture network expansion, the horizontal stress difference of the reservoir, the main fracture spacing and the number of main fracture clusters is specifically as follows:

[0012] The smaller the distance between the main fractures, the more obvious the influence of the stress interference between the fractures, and the weaker the ability of the main fractures to penetrate the natural fractures and extend to the deep formation. When the distance between the fractures is greater than 40m, the expansion of the fracture network is not affected by the interference stress.

[0013] The greater the horizontal stress difference of the reservoir, the lower the curvature of the main fracture, and the stronger its ability to penetrate the natural fracture and expand to the deep reservoir. When the stress difference is greater than 8MPa, the expansion of the main fracture is almost unaffected by the interference stress.

[0014] The more main crack clusters there are, the stronger the stress interference is, and the main cracks initiate in sequence. The order of initiation is that the outermost main crack initiates and expands first, followed by the middle crack, and finally the second outer crack expands.

[0015] Furthermore, the specific influence of the interference stress between the main crack clusters is:

[0016] In the early stage of fracturing, the interference stress between the main fractures is small, and the main fractures start and expand evenly;

[0017] When the main fracture expands to a certain length, the influence of interference stress becomes larger. If the reservoir is homogeneous, the middle fracture is first suppressed and stops expanding, while the fractures on both sides expand evenly.

[0018] If the reservoir is inhomogeneous, each main fracture is affected by the interference stress and starts to crack in sequence. The expansion order is the side fractures, the middle fractures, and the secondary outer fractures.

[0019] If the main fracture intersects with the natural fracture, the interference stress will change their interaction mode, that is, when the direction of the natural fracture deviates from the direction of the hydraulic fracture, the hydraulic fracture is more likely to penetrate the natural fracture. On the contrary, when the directions of the natural fracture and the hydraulic fracture are close, the hydraulic fracture is more likely to open the natural fracture and expand along the direction of the natural fracture.

[0020] The specific influence of the main fracture spacing is as follows: under a reservoir stress difference of 4MPa, when the number of main fracture clusters is 3, when the spacing between main fractures is less than 40m, the smaller the cluster spacing, the greater the influence of the interference stress between fractures, and the fractures on both sides will deviate from the middle fracture to expand, while the middle fracture is suppressed, resulting in its fracture expansion depth being significantly lower than the fractures on both sides, and the fracture morphology is more tortuous; when the fracture spacing exceeds the threshold of 40m, the influence of the interference stress between the main fractures is weakened, and each main fracture starts to fracture in turn in each temporary plugging stage, and the fracture expansion depth and morphology tend to be consistent; within the fracture plane, the influence of the interference stress will increase the expansion resistance of the fracture, causing the middle fracture to be short and wide, and the fractures on both sides to be narrow and long.

[0021] The influence of the horizontal stress difference is as follows: when the spacing between the crack clusters is 20m and the number of main crack clusters is 3, as the horizontal stress difference increases, the stress interference between the main crack clusters weakens, the probability of crack deflection decreases, and the main cracks are more likely to penetrate the natural cracks and expand along the direction of the maximum horizontal principal stress. Under different stress differences, the cracking order of each cluster does not change. The main cracks preferentially start from the cracks on both sides, followed by the middle crack and the secondary outer crack.

[0022] Furthermore, the influence of the number of main fracture clusters is as follows: under certain cluster spacing and horizontal stress difference, as the number of perforation clusters increases, the degree of stress interference between fractures intensifies, and due to the uneven distribution of natural fractures, the main fractures deviate and expand toward each other to varying degrees;

[0023] When the main cracks are close to each other or connected, their relative distance decreases, the interference stress increases, the interference between cracks increases, the crack width decreases in the area close to the main cracks, and tends to close;

[0024] When the main cracks diverge and expand, the interference between cracks weakens, the main cracks tend to expand evenly, and the expansion and extension resistance decreases;

[0025] The more perforation clusters there are, the more temporary plugging times are required, the more complex the main fracture propagation trajectory is, and the more complex the fracture network system is.

[0026] Furthermore, the two-dimensional horizontal well fracturing complex fracture propagation model composed of natural fractures, hydraulic fractures and rock matrix is:

[0027] Dynamic allocation of temporary plugging fracturing flow: the impact of stress interference on the flow rate of each main fracture and fracture expansion during temporary plugging fracturing of horizontal wells;

[0028] Natural fracture distribution: the distribution of natural fractures in shale reservoirs and the influence of physical and mechanical properties on complex fracture morphology;

[0029] Fluid-solid coupling: the flow state of fracturing fluid in fractures (tangential flow and normal seepage) and the influence of seepage in rock matrix on rock skeleton and porosity and permeability characteristics;

[0030] Fracture initiation: Rock cracks begin when the bottom hole pressure reaches the tensile and shear strength of the rock;

[0031] Crack expansion and extension: After the rock cracks, when its energy release rate reaches the rock's inherent critical energy release rate, the crack extends forward;

[0032] Furthermore, the dynamic allocation of temporary plugging fracturing flow rate includes the establishment of perforation units, fluid continuity equations, and finite element form of fluid pressure drop based on ABAQUS-UEL;

[0033] The specific method is:

[0034] The newly developed perforation unit contains only two pore pressure nodes describing the fluid pore pressure, which are used to describe the pore pressure changes at the injection end and the outflow end. The pressure drop between the two points of the perforation unit is proportional to the square of the injection flow rate, and the continuity equation of the fluid can be expressed as:

[0035]

[0036] Where I is the number of perforation clusters; The fluid pressure drop at both ends of the perforation cluster I; p1 and p2 are the pressures at the inlet and outlet of the perforation unit, respectively; Q I is the injection rate of perforation cluster I; n p , D p and C are the number of perforations, perforation diameter and dimensionless compensation coefficient of perforation cluster I, respectively; ρ is the fluid density,; is about ρ, n p , D p and the parameter expression of C.

[0037] The fluid continuity equation is rewritten into finite element form using the self-compiled unit subroutine of ABAQUS-UEL, which is specifically expressed as follows:

[0038]

[0039] Where [p1 p2] T is the node pore pressure matrix; [q1 q2] T Nodal fluid flow matrix; is about ρ, n p , D p and the parameter expression of C, p1 and p2 are the pressures at the inlet and outlet of the perforation unit respectively; related to the perforation diameter, fluid density and dimensionless compensation coefficient; the finite element form is solved iteratively by the Newton-Raphson method;

[0040] If there are multiple clusters of main fractures, multiple self-organized perforation units share a fluid injection node, but each has a fluid outflow node.

[0041] Further, the natural fracture distribution includes a natural fracture insertion method and a cohesive fracture unit embedding method;

[0042] The distribution of natural fractures is as follows:

[0043] The length and distribution angle of natural fractures in real reservoirs are random, so Python is used to establish a pre-processing model of randomly distributed natural fractures, and its distribution length and angle can be randomly adjusted;

[0044] On the basis of the pre-processing model, a self-compiled python subroutine is used to globally insert cohesive pore pressure units, namely cohesive units, at the boundary of unit rock units based on the rock fracture and damage theory. On the basis of the traditional finite element model, the random expansion of hydraulic fractures and the interaction simulation between natural fractures are realized.

[0045] Furthermore, the fluid-solid coupling process includes the stress balance equation of the rock porous medium, the fluid continuity equation and the fluid flow equation in the fracture;

[0046] Among them, the stress balance equation of rock porous media is:

[0047] The stress of rock is affected by the wetting fluid pressure p w , average pressure of non-wetting fluid p a and effective stress The influence of is expressed as follows:

[0048]

[0049] Wherein χ is the surface tension factor, when the rock is completely saturated, χ=1.0, and in the present invention, χ depends on the saturation of the rock; I is the unit matrix, and σ is the rock stress.

[0050] During the hydraulic fracturing process, as the fracturing fluid enters, the rock pores begin to absorb the fluid and expand, so the effective stress is composed of the compressive stress of the absorbed fluid and the rock skeleton stress. According to the simplification, the effective stress expression is:

[0051]

[0052] In the formula, is the effective stress, is the effective stress of the rock skeleton, is the average compressive stress inside the suction liquid, n t is the porosity and I is the unit matrix.

[0053] For fully saturated rock, and without considering the absorption of fracturing fluid into rock pores, the effective stress formula can be simplified to:

[0054]

[0055] In the formula, is the effective stress, is the effective stress of the rock skeleton, p w is the stress of rock subjected to the wetting fluid pressure; I is the unit matrix.

[0056] According to the principle of virtual work, the equilibrium equation of the rock skeleton at any time can be expressed as:

[0057]

[0058] in, and δε are effective stress and virtual strain respectively, δv is virtual displacement, τ and f are surface force per unit area and body force per unit volume respectively, p w is the stress of rock subjected to the wetting fluid pressure, I is the unit matrix, S is the calculation area, and V is the calculation volume.

[0059] Continuity equation for the upper fluid:

[0060] The continuous flow of fluid in porous rock matrix conforms to Darcy flow. According to the continuity equation of fluid, the change of fluid mass is equal to the mass of fluid passing through the porous medium per unit time. Its mass conservation equation can be expressed as:

[0061]

[0062] According to Gauss's formula, the fluid continuity equation can be obtained as follows:

[0063]

[0064] Where J is the rate of change of porous media, ρ w , n w and v w are the density of the fluid, the porosity of the rock medium and the average velocity of the fluid, respectively; x is the space vector, V is the fluid volume, t is the calculation time, S is the fluid flow area, and n is the average velocity of the fluid. T is the external normal vector of S.

[0065] The flow of fluid in porous media is described by Darcy's law, which expresses the relationship between fluid seepage velocity and pressure drop. The expression is as follows:

[0066]

[0067] In the formula, ρ w , n w and v ware the density of the fluid, the porosity of the rock medium and the average velocity of the fluid, k and g are the permeability coefficient and gravitational acceleration, respectively, and p w is the pore pressure.

[0068] Fluid flow in cracks, namely tangential seepage and normal seepage:

[0069] The fluid in the crack is assumed to be an incompressible Newtonian fluid, and the flow velocity of the tangential flow is calculated by the lubrication equation, and its mass conservation equation is written as follows:

[0070]

[0071] Where q is the velocity of the tangential flow, w is the crack width, and μ is the viscosity of the fracturing fluid. is the pressure drop in the tangent direction of the seam.

[0072] At the same time, the cohesion model also considers the seepage from the fracture wall to the rock matrix, that is, the normal filtration, which is expressed as:

[0073] q t =c t (p i -p t )

[0074] q b =c b (p i -p b )

[0075] In the formula, q t and q b are the normal filtration velocity of the upper and lower walls of the unit, c t and c b are the filtration coefficients of the upper and lower walls, respectively, and p t and p b are the pore pressures on the upper and lower walls, respectively, and p i is the pore pressure at the middle pore pressure node of the fracture unit.

[0076] According to the mass equation of fluid flow in the crack, the continuity equation of fluid flow in the crack can be expressed as:

[0077]

[0078] w is the crack width, μ is the viscosity of the fracturing fluid, is the pressure drop in the tangent direction of the seam, q t and q b are the normal filtration velocity of the upper and lower walls of the unit, c t and c b are the filtration coefficients of the upper and lower walls, respectively, and p t and pb are the pore pressures on the upper and lower walls, respectively, and p i is the pore pressure of the middle pore pressure node of the fracture unit, Q is the fluid flow rate, t is the fluid circulation time, and δ(x, y) is the Dirac δ function.

[0079] Furthermore, the crack initiation criterion for the crack initiation adopts a secondary nominal stress criterion that can describe tension and shear failure, and its expression is as follows:

[0080]

[0081] Where t n , t s and t t Represent the cohesive unit normal stress and shear stress in two directions respectively; and They represent the normal critical stress of the cohesive unit, that is, the tensile strength and the critical stresses in the two tangential directions respectively.

[0082] Furthermore, the crack expansion and extension is the damage evolution process of the material after the material-specific crack initiation criteria are met:

[0083] The crack propagation model in the crack propagation and extension adopts the Benzeggagh-Kenane energy criterion to describe the damage evolution process of the unit. When the first tangential fracture energy and the second tangential fracture energy are equal, the expression of the Benzeggagh-Kenane energy criterion is:

[0084]

[0085] Where η is the material constant, G c is the total fracture energy during the damage evolution process, where G n , G s and G t denote the fracture energy in the element normal direction and two tangential directions, respectively, and represent the critical energy release rate in the element normal and tangential directions, respectively.

[0086] Beneficial effects:

[0087] The present invention has the following beneficial effects:

[0088] This prediction method is based on rock damage and fracture mechanics theory, and numerical simulation of fracture expansion in temporary plugging fracturing of horizontal wells in shale reservoirs was carried out on the ABAQUS software platform. The dynamic distribution process of fracturing fluid flow under stress interference between main fractures during temporary plugging fracturing was considered, and the effects of perforation cluster spacing, perforation cluster number, and horizontal stress difference on the competitive fracturing sequence of main fractures and fracture expansion morphology were analyzed. The relationship between fracture expansion morphology, competitive fracturing sequence of main fractures, and fracture network expansion morphology under different construction parameters and ground stress conditions was obtained.

[0089] The model can be used to obtain the optimal design values ​​of the number of perforation clusters and the spacing between perforation clusters under different stress conditions, which can be used to guide the design of on-site temporary plugging and fracturing schemes. According to the expected crack morphology, the crack extension length, depth and effective transformation volume of the fracture network can be calculated, and then the fracturing construction parameters can be determined. Description of the drawings:

[0090] Figure 1 This is a diagram of the temporary plugging and fracturing structure of a horizontal well in a shale reservoir in an embodiment of the present application;

[0091] Figure 2 This is a structural diagram of a newly developed perforation unit in the embodiment of the present application;

[0092] Figure 3 This is a schematic diagram of the structure of a perforation cluster unit in an embodiment of the present application;

[0093] Figure 4 This is a diagram of the adjacent structures of the rock matrix unit and the cohesive unit in the embodiment of the present application;

[0094] Figure 5 This is a diagram of the fluid flow pattern in the Cohesive fracture unit in the embodiment of the present application;

[0095] Figure 6 The damage evolution criterion of the energy release rate of the Cohesive unit in the embodiment of the present application;

[0096] Figure 7 The expansion law of complex seam network under different horizontal stress differences in the embodiment of this application

[0097] Figure 8 The expansion law of complex seam network under different horizontal stress differences in the embodiment of this application

[0098] Fig. 9 The expansion law of complex seam network under different horizontal stress differences in the embodiment of this application

[0099] Fig.10 The expansion law of complex seam network under different horizontal stress differences in the embodiment of this application

[0100] Fig.11 The expansion law of complex seam network under different cluster spacing in the embodiment of this application (S=10m);

[0101] Fig.12 The expansion law of complex seam network under different cluster spacing in the embodiment of this application (S=20m);

[0102] Fig.13 The expansion law of complex seam network under different cluster spacing in the embodiment of this application (S=30m);

[0103] Fig.14 The expansion law of complex seam network under different cluster spacing in the embodiment of this application (S=40m);

[0104] Fig.15 The expansion law of complex fracture network under different numbers of perforation clusters in the embodiment of this application (N=3 clusters);

[0105] Fig.16 The expansion law of complex fracture network under different numbers of perforation clusters in the embodiment of the present application (N=4 clusters);

[0106] Fig.17 The expansion law of complex fracture network under different numbers of perforation clusters in the embodiment of this application (N=5 clusters);

[0107] Fig.18 The expansion law of complex fracture network under different perforation cluster numbers in the embodiment of this application (N = 6 clusters). DETAILED DESCRIPTION

[0108] The present invention is further described below in conjunction with the accompanying drawings, and the protection scope of the present invention is not limited to the following:

[0109] like Figure 1 As shown, it is a structural diagram of temporary plugging and fracturing of a horizontal well in a shale reservoir in an embodiment of the present application, and a method for predicting competitive fracturing of multiple fractures by temporary plugging and fracturing of a horizontal well in a shale reservoir, specifically comprising:

[0110] A two-dimensional horizontal well fracturing complex fracture propagation finite element model consisting of natural fractures, hydraulic fractures and rock matrix was established based on the geological data of the main strata of a deep shale gas reservoir in southern Sichuan. The model size is 120×100m, the number of perforation clusters is 2-5, the perforation cluster spacing is 10-40m, and the perforation direction is along the horizontal maximum principal stress direction. The basic parameters of the model are shown in Table 1:

[0111] Table 1 Basic parameters of shale reservoir model

[0112]

[0113]

[0114] like Figure 2 As shown in Figure 1, a perforation unit is established to realize the dynamic distribution of fracturing fluid flow. This unit contains only two pore pressure nodes, which can describe the changes in the inlet and outlet pressures of the fracturing fluid. If there are multiple clusters of perforations, the multiple clusters of perforations share one pore pressure node to describe the relationship between the inlet pressure changes under the interference stress. Its structure is as follows: Figure 3 As shown. The continuity equation of the perforating unit fracturing fluid is:

[0115]

[0116] Where I is the number of perforation clusters; the fluid pressure drop at both ends of perforation cluster I; p1 and p2 are the pressures at the inlet and outlet of the perforation unit, respectively; is the injection flow rate of perforation cluster I; n p , D p and C are the number of perforations, perforation diameter and dimensionless compensation coefficient of perforation cluster I, respectively; ρ is the fluid density; is about ρ, n p , D p and the parameter expression of C.

[0117] Based on the damage fracture mechanics theory, a cohesive unit with pore pressure nodes is globally inserted into the rock matrix model. All rock intersection boundaries are inserted using a Python self-written subroutine to simulate the random expansion of discrete fractures within the finite element framework. At the same time, a collection of natural fracture and hydraulic fracture units is established to achieve differentiated input of different fracture parameters. The local rock fracture structure with cohesive units inserted is shown in Figure 2. Figure 4 shown.

[0118] The basic model and the perforation unit model are combined, and the ABAQUS-Standard solver is used to define the initial geostress, gas saturation, rock porosity, pore pressure and other values ​​in the initial boundary conditions. At the beginning of the solution, the geostress balance (Geostatic) analysis step is used to solve the geostress balance, and the implicit geotechnical module (Standard-Soil) is used for iterative solution in the subsequent temporary plugging process. Perforation units are inserted into the horizontal well section according to different cluster spacings. Multiple clusters of perforation units share a pore pressure node to simulate the dynamic distribution process of multi-cluster perforation fracturing fluid flow. All models are divided by quadrilateral structure grids of approximate size to reduce the model error caused by size differences. At the same time, the plane stress pore pressure unit (CPE4P) is used to simulate the pore pressure and permeability characteristics of the rock matrix.

[0119] The establishment of the fluid-solid coupling hydraulic fracturing fracture extension model includes:

[0120] Rock matrix stress balance equation:

[0121] The stress of rock is affected by the wetting fluid pressure (p w ), the average pressure of the non-wetting fluid (p a ) and effective stress The influence of is expressed as follows:

[0122]

[0123] Wherein χ is the surface tension factor, when the rock is completely saturated, χ=1.0, and in the present invention, χ depends on the saturation of the rock; I is the unit matrix, and σ is the rock stress.

[0124] During the hydraulic fracturing process, as the fracturing fluid enters, the rock pores begin to absorb the fluid and expand, so the effective stress is composed of the compressive stress of the absorbed fluid and the rock skeleton stress. According to the simplification, the effective stress expression is:

[0125]

[0126] In the formula is the effective stress of the rock skeleton, is the average compressive stress inside the suction liquid, n t is the porosity.

[0127] For fully saturated rock, and without considering the absorption of fracturing fluid into rock pores, the effective stress formula can be simplified to:

[0128]

[0129] According to the principle of virtual work, the equilibrium equation of the rock skeleton at any time can be expressed as:

[0130]

[0131] in, and δε are effective stress and virtual strain respectively, δv is virtual displacement, τ and f are surface force per unit area and body force per unit volume respectively, p w is the stress of rock subjected to the wetting fluid pressure, I is the unit matrix, S is the calculation area, and V is the calculation volume.

[0132] Fluid continuity equation:

[0133] The continuous flow of fluid in porous rock matrix conforms to Darcy flow. According to the continuity equation of fluid, the change of fluid mass is equal to the mass of fluid passing through the porous medium per unit time. Its mass conservation equation can be expressed as:

[0134]

[0135] According to Gauss's formula, the fluid continuity equation can be obtained as follows:

[0136]

[0137] Where J is the rate of change of porous media, ρ w , n w and v w are the density of the fluid, the porosity of the rock medium and the average velocity of the fluid, respectively; x is the space vector, V is the fluid volume, t is the calculation time, S is the fluid flow area, and n is the average velocity of the fluid. T is the external normal vector of S.

[0138] The flow of fluid in porous media is described by Darcy's law, which expresses the relationship between fluid seepage velocity and pressure drop. The expression is as follows:

[0139]

[0140] Where k and g are the permeability coefficient and gravitational acceleration, respectively, p w is the pore pressure.

[0141] like Figure 5 As shown, fluid flows in cracks (tangential seepage and normal seepage):

[0142] The fluid in the crack is assumed to be an incompressible Newtonian fluid, and the velocity of the tangential flow is calculated by the lubrication equation. Its mass conservation equation can be written as follows:

[0143]

[0144] Where q is the velocity of the tangential flow, w is the crack width, μ is the viscosity of the fracturing fluid, is the pressure drop in the tangent direction of the fracture. At the same time, the cohesion model also considers the seepage (normal filtration) from the fracture wall to the rock matrix, and its expression is:

[0145]

[0146] Where q t and q b are the normal filtration velocity of the upper and lower walls of the unit, c t and c b are the filtration coefficients of the upper and lower walls, respectively, and p t and p b are the pore pressures on the upper and lower walls, respectively, and p i is the pore pressure at the middle pore pressure node of the fracture unit.

[0147] According to the mass equation of fluid flow in the crack, the continuity equation of fluid flow in the crack can be expressed as:

[0148]

[0149] w is the crack width, μ is the viscosity of the fracturing fluid, is the pressure drop in the tangent direction of the seam, q t and q b are the normal filtration velocity of the upper and lower walls of the unit, c t and c b are the filtration coefficients of the upper and lower walls, respectively, and p t and p b are the pore pressures on the upper and lower walls, respectively, and p i is the pore pressure of the middle pore pressure node of the fracture unit, Q is the fluid flow rate, t is the fluid circulation time, and δ(x, y) is the Dirac δ function.

[0150] The fracture initiation criterion of the temporary plugging and fracturing fracture propagation model of the shale reservoir horizontal well adopts the secondary nominal stress criterion that can describe the tension and shear failure, and its expression is as follows:

[0151]

[0152] Where t n , t s and t t Represent the cohesive unit normal stress and shear stress in two directions respectively; and They represent the normal critical stress (tensile strength) of the cohesive element and the critical stresses in the two tangential directions respectively.

[0153] The crack extension and propagation is the damage evolution process of the material after the material-specific crack initiation criteria are met:

[0154] like Figure 6 As shown in the figure, the fracture propagation model of temporary plugging and fracturing in horizontal wells of shale reservoirs adopts the Benzeggagh-Kenane (BK) energy criterion to describe the damage evolution process of the unit. When the first tangential fracture energy and the second tangential fracture energy are equal, the BK criterion expression is:

[0155]

[0156] Where η is the material constant, G c is the total fracture energy during the damage evolution process, where G n , G s and G t denote the fracture energy in the element normal direction and two tangential directions, respectively, and represent the critical energy release rate in the element normal and tangential directions, respectively.

[0157] Based on the hydraulic fracturing finite element model, the factors affecting the competitive fracturing sequence and fracture morphology of temporary plugging fracturing in horizontal wells in shale reservoirs are analyzed, including the horizontal stress difference of the reservoir, the number of perforation clusters, and the spacing between perforation clusters.

[0158] like Figure 7-10 , the relationship between the competitive cracking sequence of the main cracks and the crack extension morphology under different horizontal stress differences is:

[0159] With the increase of horizontal stress difference, the influence of interference stress between temporary plugging and fracturing cracks is weakened, the probability of crack deflection is reduced, and the cracks are more inclined to penetrate natural cracks and expand along the direction of horizontal maximum principal stress, and the crack morphology tends to be single;

[0160] Under different horizontal stress differences, the order of competitive cracking between the main cracks basically does not change, and the cracks on both sides expand first, followed by the expansion of the middle crack;

[0161] When the horizontal stress difference is low, the interference between fractures is more obvious, and each cluster of fractures undergoes obvious turning, which is manifested as a large degree of tortuosity in the near-wellbore area, and even interconnection occurs, which will lead to a reduction in the reservoir transformation volume. In addition, when the ground stress difference is low (lower than 4MPa), the bottom hole initiation pressure is low, the fracture extension resistance is small, and it is easy to initiate fractures along the direction of natural fractures, which will increase the reservoir transformation volume. When the reservoir stress difference is high (higher than 6MPa), most hydraulic fractures pass through natural fractures to form a straight single main fracture;

[0162] For low stress difference reservoirs, in the early stage of fracturing, the main fracture extension and fracturing pressure can be increased by increasing the injection displacement and viscosity of the fracturing fluid, so that the main fracture can pass through more natural fractures near the wellbore, overcome stress interference, and extend to the deep part of the reservoir. In the later stage of fracturing, the fracturing fluid displacement and viscosity can be reduced to reduce the fracture extension pressure, and more natural fractures can be connected by stress interference, thereby increasing the reservoir transformation volume.

[0163] For reservoirs with large stress differences, the cluster spacing is often reduced and the number of perforation clusters is increased to increase stress interference, causing fractures to deflect deep in the reservoir, thereby maximizing the communication of natural fractures and increasing the reservoir transformation volume.

[0164] like Figure 11-Figure 14 The relationship between the main fracture competition initiation sequence and the fracture extension morphology under different perforation cluster spacings is:

[0165] Due to the heterogeneity of shale reservoirs and the interference of induced stress between fractures during fracturing, the order of fracture initiation is inconsistent under different cluster spacing during temporary plugging and fracturing. When the stress interference between fractures is strong, the fracture clusters on both sides initiate first, and the middle fracture cluster initiates and expands last.

[0166] The larger the distance between the main crack clusters, the smaller the influence of the inter-crack interference stress, and the crack morphology tends to be simpler;

[0167] Temporary plugging fracturing often uses low cluster spacing fracturing. Due to the influence of interference stress, the main fractures will be over-deflected or interconnected, resulting in a reduction in the effective length of the fracture network. Secondly, the greater the interference stress, the greater the resistance to the extension of the main fractures. The allocated fracturing fluid mainly acts to open the fractures. The fractures are short and wide and cannot extend to the deep part of the reservoir, resulting in a reduction in the reservoir transformation volume.

[0168] When the perforation cluster spacing exceeds 40 m, the interference stress between fractures has little effect, and each cluster of fractures expands along the direction of the maximum horizontal principal stress, interacting with the natural fractures to form a uniformly expanded fracture network system.

[0169] like Figure 15-18 The relationship between the competitive cracking sequence of the main fractures and the crack extension morphology under different numbers of perforation clusters is:

[0170] When multiple clusters of fractures expand, the fractures on both sides tend to expand first, and then expand to the middle fracture in sequence, indicating that when multiple clusters of fractures expand simultaneously in horizontal well fracturing, the stress interference of the middle fracture is the most serious;

[0171] When the cluster spacing is constant, the number of perforation clusters is large, and the subsequent fracturing cracks extend to a lower depth in the reservoir due to strong stress interference. The subsequent fracturing crack clusters are more likely to form a fracture network near the wellbore area, and the effective volume of the fracturing cracks is reduced;

[0172] In temporary plugging fracturing, although increasing the number of perforation clusters can increase the efficiency of fracture expansion in each cluster, when the cluster spacing is small, the efficiency of the fracture network formation of the middle fracture cluster is reduced due to stress interference. Therefore, the relationship between the number of perforation clusters and the cluster spacing needs to be comprehensively considered in the field fracturing plan.

[0173] In summary, the prediction method disclosed in the present invention is based on the rock damage and fracture mechanics theory, and a numerical simulation study of the crack expansion of temporary plugging fracturing in horizontal wells in shale reservoirs is conducted on the ABAQUS limited software platform. The dynamic distribution process of fracturing fluid flow under the stress interference between the main fractures during temporary plugging fracturing is considered, and the influence of the perforation cluster spacing, the number of perforation clusters, the horizontal stress difference and the natural fractures on the competitive fracturing sequence of the main fractures and the crack expansion morphology are analyzed, and the relationship between the crack expansion morphology, the competitive fracturing sequence of the main fractures and the fracture network expansion morphology under different construction parameters and ground stress conditions is obtained.

[0174] The model can be used to obtain the optimal design values ​​of the number of pore clusters and the spacing between perforation clusters under different stress conditions, which can be used to guide the design of on-site temporary plugging and fracturing schemes. According to the expected crack morphology, the crack extension length, depth and effective transformation volume of the fracture network can be calculated, and then the fracturing construction parameters can be determined.

Claims

1. A method for predicting competitive fracturing of multiple fractures during temporary plugging and fracturing of horizontal wells in shale reservoirs, characterized in that: Specifically include: Establish a two-dimensional horizontal well fracturing complex fracture network expansion model consisting of natural fractures, hydraulic fractures and rock matrix; Based on the horizontal well hydraulic fracturing fracture propagation model, the factors affecting the competitive fracturing and fracture network expansion of shale reservoir fracturing are analyzed, including the influence of interference stress between fracture clusters, the main fracture spacing, the number of main fracture clusters, and the horizontal stress difference; The relationship between the initiation sequence of multiple fractures, the complexity of fracture network expansion, the horizontal stress difference of the reservoir, the spacing between main fractures and the number of main fracture clusters was analyzed. The relationship between the sequence of multiple fracture initiation, the complexity of fracture network expansion, the horizontal stress difference of the reservoir, the spacing between main fractures and the number of main fracture clusters is used as a prediction method for competitive fracturing and complex fracture network expansion in horizontal wells of shale reservoirs. The two-dimensional horizontal well fracturing complex fracture propagation model composed of natural fractures, hydraulic fractures and rock matrix is: Dynamic allocation of temporary plugging fracturing flow: the impact of stress interference on the flow rate of each main fracture and fracture expansion during temporary plugging fracturing of horizontal wells; Natural fracture distribution: the distribution of natural fractures in shale reservoirs and the influence of physical and mechanical properties on complex fracture morphology; Fluid-solid coupling: the influence of tangential flow and normal seepage in the flow state of fracturing fluid and seepage in rock matrix on rock skeleton and porosity and permeability characteristics; Fracture initiation: Rock cracks begin when the bottom hole pressure reaches the tensile and shear strength of the rock; Crack expansion and extension: After the rock cracks, when its energy release rate reaches the rock's inherent critical energy release rate, the crack extends forward; The fluid-solid coupling process includes the stress balance equation of rock porous media, the fluid continuity equation and the fluid flow equation model in the fracture; Among them, the stress balance equation of rock porous media is: The stress of rock is affected by the wetting fluid pressure p w , average pressure of non-wetting fluid p a and effective stress The influence of is expressed as follows: Where χ is the surface tension factor, χ = 1.0 when the rock is fully saturated, and χ depends on the saturation of the rock; I is the unit matrix, and σ is the rock stress; During the hydraulic fracturing process, as the fracturing fluid enters, the rock pores begin to absorb the fluid and expand, so the effective stress is composed of the compressive stress of the absorbed fluid and the rock skeleton stress. According to the simplification, the effective stress expression is: In the formula, is the effective stress, is the effective stress of the rock skeleton, is the average compressive stress inside the suction liquid, n t is the porosity, I is the unit matrix; For fully saturated rock, and without considering the absorption of fracturing fluid into rock pores, the effective stress formula can be simplified to: In the formula, is the effective stress, is the effective stress of the rock skeleton, p w is the stress of rock subjected to the wetting fluid pressure; I is the unit matrix; According to the principle of virtual work, the equilibrium equation of the rock skeleton at any time can be expressed as: in, and δε are effective stress and virtual strain respectively, δv is virtual displacement, τ and f are surface force per unit area and body force per unit volume respectively, p w is the stress of rock subjected to the wetting fluid pressure, I is the unit matrix, S is the calculation area, and V is the calculation volume; Fluid continuity equation: The continuous flow of fluid in porous rock matrix conforms to Darcy flow. According to the continuity equation of fluid, the change of fluid mass is equal to the mass of fluid passing through the porous medium per unit time. Its mass conservation equation can be expressed as: According to Gauss's formula, the fluid continuity equation can be obtained as follows: Where J is the rate of change of porous media, ρ w , n w and v w are the density of the fluid, the porosity of the rock medium and the average velocity of the fluid, respectively; x is the space vector, V is the fluid volume, t is the calculation time, S is the fluid flow area, and n is the average velocity of the fluid. T is the external normal vector of S; The flow of fluid in porous media is described by Darcy's law, which expresses the relationship between fluid seepage velocity and pressure drop. The expression is as follows: In the formula, ρ w , n w and v w are the density of the fluid, the porosity of the rock medium and the average velocity of the fluid, k and g are the permeability coefficient and gravitational acceleration, respectively, and p w is the pore pressure; Fluid flow in cracks, namely tangential seepage and normal seepage: The fluid in the crack is assumed to be an incompressible Newtonian fluid, and the flow velocity of the tangential flow is calculated by the lubrication equation, and its mass conservation equation is written as follows: Where q is the velocity of the tangential flow, w is the crack width, and μ is the viscosity of the fracturing fluid. is the pressure drop in the tangent direction of the seam; At the same time, the cohesion model also considers the seepage from the fracture wall to the rock matrix, that is, the normal filtration, which is expressed as: q t =c t (p i -p t ) q b =c b (p i -p b ) In the formula, q t and q b are the normal filtration velocity of the upper and lower walls of the unit, c t and c b are the filtration coefficients of the upper and lower walls, respectively, and p t and p b are the pore pressures on the upper and lower walls, respectively, and p i is the pore pressure of the middle pore pressure node of the fracture unit; According to the mass equation of fluid flow in the crack, the continuity equation of fluid flow in the crack can be expressed as: w is the crack width, μ is the viscosity of the fracturing fluid, is the pressure drop in the tangent direction of the seam, q t and q b are the normal filtration velocity of the upper and lower walls of the unit, c t and c b are the filtration coefficients of the upper and lower walls, respectively, and p t and p b are the pore pressures on the upper and lower walls, respectively, and p i is the pore pressure of the middle pore pressure node of the fracture unit, Q is the fluid flow rate, t is the fluid circulation time, and δ(x, y) is the Dirac δ function.

2. A method for predicting competitive fracturing of multiple fractures in a shale reservoir horizontal well by temporary plugging and fracturing as claimed in claim 1, characterized in that: The relationship between the initiation sequence of multiple fractures, the complexity of fracture network expansion, the horizontal stress difference of the reservoir, the main fracture spacing and the number of main fracture clusters is specifically as follows: The smaller the distance between the main fractures, the more obvious the influence of the stress interference between the fractures, and the weaker the ability of the main fractures to penetrate the natural fractures and extend to the deep formation. When the distance between the fractures is greater than 40m, the expansion of the fracture network is not affected by the interference stress. The greater the horizontal stress difference of the reservoir, the lower the curvature of the main fracture, and the stronger its ability to penetrate the natural fracture and expand to the deep reservoir. When the stress difference is greater than 8MPa, the expansion of the main fracture is almost unaffected by the interference stress. The more main crack clusters there are, the stronger the stress interference is, and the main cracks initiate in sequence. The order of initiation is that the outermost main crack initiates and expands first, followed by the middle crack, and finally the second outer crack expands.

3. A method for predicting competitive fracturing of multiple fractures in a shale reservoir horizontal well by temporary plugging and fracturing as claimed in claim 1, characterized in that: The specific influence of the interference stress between the main crack clusters is: In the early stage of fracturing, the interference stress between the main cracks is smaller than that in the later stage of fracturing, and the main cracks start and expand evenly; When the main fracture extension length exceeds the perforation length, the interference stress becomes more influential. If the reservoir is homogeneous, the middle fracture is first suppressed and stops expanding, while the fractures on both sides expand evenly. If the reservoir is inhomogeneous, each main fracture is affected by the interference stress and starts to crack in sequence. The expansion order is the side fractures, the middle fractures, and the secondary outer fractures. If the main fracture intersects with the natural fracture, the interference stress will change their interaction mode, that is, when the direction of the natural fracture deviates from the direction of the hydraulic fracture, the hydraulic fracture is more likely to penetrate the natural fracture. On the contrary, when the directions of the natural fracture and the hydraulic fracture are close, the hydraulic fracture is more likely to open the natural fracture and expand along the direction of the natural fracture. The specific influence of the main fracture spacing is as follows: under a reservoir stress difference of 4MPa, when the number of main fracture clusters is 3, when the spacing between main fractures is less than 40m, the smaller the cluster spacing, the greater the influence of the interference stress between fractures, and the fractures on both sides will deviate from the middle fracture to expand, and the middle fracture will be suppressed, resulting in its fracture expansion depth being significantly lower than the fractures on both sides, and the fracture morphology is more tortuous; when the fracture spacing exceeds the threshold of 40m, the influence of the interference stress between the main fractures is weakened, and each main fracture starts to crack in turn in each temporary plugging stage, and the fracture expansion depth and morphology tend to be consistent; in the fracture plane, the influence of the interference stress will increase the expansion resistance of the fracture, causing the middle fracture to be short and wide, and the fractures on both sides to be narrow and long; The influence of the horizontal stress difference is as follows: when the spacing between crack clusters is 20m and the number of main crack clusters is 3, as the horizontal stress difference increases, the stress interference between the main crack clusters weakens, the probability of crack deflection decreases, and the main cracks are more inclined to penetrate the natural cracks and expand along the direction of the maximum horizontal principal stress; under different stress differences, the cracking order of each cluster of cracks does not change, and the main cracks preferentially initiate from the cracks on both sides, followed by the middle crack and the secondary outer crack.

4. A method for predicting competitive fracturing of multiple fractures in a shale reservoir horizontal well by temporary plugging and fracturing as claimed in claim 1, characterized in that: The influence of the number of main fracture clusters is as follows: under fixed cluster spacing and horizontal stress difference, as the number of perforation clusters increases, the degree of stress interference between fractures intensifies. Due to the uneven distribution of natural fractures, the main fractures deviate and expand toward each other to varying degrees. When the main cracks are close to each other or connected, their relative distance decreases, the interference stress increases, the interference between cracks increases, the crack width decreases in the area close to the main cracks, and tends to close; When the main cracks diverge and expand, the interference between cracks weakens, the main cracks tend to expand evenly, and the expansion and extension resistance decreases; The more perforation clusters there are, the more temporary plugging times are required, the more complex the main fracture propagation trajectory is, and the more complex the fracture network system is.

5. A method for predicting competitive fracturing of multiple fractures in a shale reservoir horizontal well by temporary plugging and fracturing as claimed in claim 1, characterized in that: The dynamic allocation of temporary plugging and fracturing flow rate includes the establishment of perforation units, fluid continuity equations, and finite element form of fluid pressure drop based on ABAQUS-UEL; The specific method is: The newly developed perforation unit contains only two pore pressure nodes describing the fluid pore pressure, which are used to describe the pore pressure changes at the injection end and the outflow end. The pressure drop between the two points of the perforation unit is proportional to the square of the injection flow rate, and the continuity equation of the fluid can be expressed as: Where I is the number of perforation clusters; The fluid pressure drop at both ends of the perforation cluster I; p1 and p2 are the pressures at the inlet and outlet of the perforation unit, respectively; Q I is the injection rate of perforation cluster I; n p , D p and C are the number of perforations, perforation diameter and dimensionless compensation coefficient of perforation cluster I; ρ is the fluid density, is about ρ, n p , D p and the parameter expression of C; The fluid continuity equation is rewritten into finite element form using the self-compiled unit subroutine of ABAQUS-UEL, which is specifically expressed as follows: Where [p1 p2] T is the node pore pressure matrix; [q1 q2] T Nodal fluid flow matrix; is about ρ, n p , D p and the parameter expression of C, p1 and p2 are the pressures at the inlet and outlet of the perforation unit respectively; related to the perforation diameter, fluid density and dimensionless compensation coefficient; the finite element form is solved iteratively by the Newton-Raphson method; If there are multiple clusters of main fractures, multiple self-organized perforation units share a fluid injection node, but each has a fluid outflow node.

6. A method for predicting competitive fracturing of multiple fractures in a shale reservoir horizontal well by temporary plugging and fracturing as claimed in claim 1, characterized in that: The natural fracture distribution includes the natural fracture insertion method and the cohesive fracture unit embedding method; The distribution of natural fractures is as follows: The length and distribution angle of natural fractures in real reservoirs are random, so Python is used to establish a pre-processing model of randomly distributed natural fractures, and its distribution length and angle can be randomly adjusted; On the basis of the pre-processing model, a self-compiled python subroutine is used to globally insert cohesive pore pressure units, namely cohesive units, at the boundary of unit rock units based on the rock fracture and damage theory. On the basis of the traditional finite element model, the random expansion of hydraulic fractures and the interaction simulation between natural fractures are realized.

7. A method for predicting competitive fracturing of multiple fractures in a shale reservoir horizontal well by temporary plugging and fracturing as claimed in claim 1, characterized in that: The crack initiation criterion for the crack initiation adopts a secondary nominal stress criterion that can describe tension and shear failure, and its expression is as follows: Where t n , t s and t t Represent the cohesive unit normal stress and shear stress in two directions respectively; and They represent the normal critical stress of the cohesive unit, that is, the tensile strength and the critical stresses in the two tangential directions respectively.

8. A method for predicting competitive fracturing of multiple fractures in a shale reservoir horizontal well by temporary plugging and fracturing as claimed in claim 1, characterized in that: The crack extension and propagation is the damage evolution process of the material after the material-specific crack initiation criteria are met: The crack propagation model in the crack propagation and extension adopts the Benzeggagh-Kenane energy criterion to describe the damage evolution process of the unit. When the first tangential fracture energy and the second tangential fracture energy are equal, the expression of the Benzeggagh-Kenane energy criterion is: Where η is the material constant, G c is the total fracture energy during the damage evolution process, G n , G s and G t denote the fracture energy in the element normal direction and two tangential directions, respectively, and and represent the critical energy release rates in the element normal and two tangential directions, respectively.

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